Copying of documents has been performed since the first recording of information in document form. Documents are produced using many procedures on many types of substrates and incorporate many forms of information. Unauthorized copying of documents has also been occurring since the storage of information in document form first began. For much of the history of information documentation, the procedures used to copy original documents have been sufficiently cumbersome and costly to provide a significant impediment to unauthorized copying, thus limiting unauthorized copying to original documents of high value. However, in more recent times the introduction of new technologies for generating reproductions of original documents has decreased the cost and inconvenience of copying documents, thus increasing the need for an effective method of inhibiting unauthorized copying of a broader range of restricted documents. The inability of convenient, low-cost copying technologies to copy original documents containing color or continuous tone pictorial information restricted unauthorized copying primarily to black-and-white documents containing textual information and line art. Recently, the introduction of cost effective document scanning and digital methods of signal processing and document reproduction have extended the ability to produce low cost copies of original documents to documents containing color and high quality pictorial information. It is now possible to produce essentially indistinguishable copies of any type of document quickly, conveniently, and cost effectively. Accordingly, the problem of unauthorized copying of original documents has been extended from simple black-and-white text to color documents, documents containing pictorial images, and photographic images. In particular, restricting the unauthorized duplication of photographic images produced by professional photographers on digital copying devices has recently become of great interest.
U.S. application Ser. No. 08/598,778, filed Feb. 8, 1996, by John Gasper, et al, titled "Copy Restrictive System" and U.S. application Ser. No. 08/598,785, filed Feb. 8, 1996, by John Gasper, et al, titled "Copy Restrictive Documents" disclose pre-exposing color photographic paper to spots of blue light to produce an array of yellow microdots after chemical processing, and a method of detecting these microdots during scanning performed by a digital printing device. Detecting these microdots in original photographs prevents unauthorized copying of the photographs. There are, however, difficulties in getting the correct pattern of microdots incorporated in photographic paper.
Methods of exposing light-sensitive photographic media for the purpose of writing an image to the media are well-known in the art. Devices that write to the media by scanning a beam of light in raster fashion across the media are called flying spot scanners. These include cathode ray tubes (CRTs) and laser scanners. The intensity of the light beam is modulated in any of a number of ways during the scanning of the beam across the media. The image being written to the media is presented to the media as a continuous signal and the image occupies the full area of the media. The duty cycle of the light source expressed as a percentage of the time the light is on and being modulated in intensity is typically 100 percent. Such scanning printers are not designed for efficiently exposing photographic media to a sparse array of microdots requiring a duty cycle of less than 10 percent and preferably less then 5 percent. Although a laser scanner can use a pulsed laser to scan an image of low duty cycle, they occupy a large volume, are expensive to build, operate, and maintain, and are designed to scan only across a limited length of scan line usually measured in inches and not feet when the writing spot size is submillimeter in diameter.
One attempt to solve this problem is disclosed in copending U.S. application Ser. No. 08/846,387, filed Apr. 30, 1997, titled "Apparatus for Creating Copy Restrictive Media", which incorporates a linear array comprised of at least two spatially distributed light sources and an aperture mask for forming two or more micro-light sources from the light sources. An optical element focuses light from the micro-light sources onto a media moving relative to the linear array. An encoding device turns the light sources on and off at regular intervals relative to movement of the media. A potential problem with this arrangement is that the optical element is located in close proximity to a rapidly moving web of material. Breaks in the web of material can scratch or damage the optical element. Also, a web dryer associated with the moving web may fail and cause the optical element to be splattered with material, thus reducing the efficiency of the optical element.
Since this type of apparatus may be producing some photographic paper which does not incorporate the copy restrictive microdots, it is often necessary to turn off the microlight sources. However, once the microlight sources are turned off, it requires a significant amount of time to warm up the microlight sources to a steady state temperature to produce microdot patterns of the right intensity. Thus, it would be desirable to incorporate microlight sources, which may remain on when photographic paper without a copy restrictive pattern is in production without exposing the photographic paper to the microlight sources. It is also desirable that this type of printhead be compact and fit into the tight confinement of existing coating machines for photographic paper without costly retrofitting and allow for the exposure of the full width of the web.